Need advice!

Okay so I have a chronic illness that makes me tired and feel sick every day basically and I really have no immune system. So my boyfriend and I have been together for a long time, but he met me before this was really affecting me badly. I'm just sexually not really feeling the mood ever, and I think it's from my illness. But we haven't had sex in 4 months. He understands and respects it, but it's something I want to do, I just don't have the drive necessarily when it comes to that time. Any advice?!? Please!